Friendship paradox

The friendship paradox is the phenomenon first observed by the sociologist Scott L. Feld in 1991 that most people have fewer friends than their friends have, on average.[1] It can be explained as a form of sampling bias in which people with more friends are more likely to be in one's own friend group. Or, said another way, one is less likely to be friends with someone who has very few friends. In contradiction to this, most people believe that they have more friends than their friends have.[2][3][4][5]

The same observation can be applied more generally to social networks defined by other relations than friendship: for instance, most people's sexual partners have had (on the average) a greater number of sexual partners than they have.[6][7]

The friendship paradox is an example of how network structure can significantly distort an individual's local observations.[8]

Mathematical explanation

In spite of its apparently paradoxical nature, the phenomenon is real, and can be explained as a consequence of the general mathematical properties of social networks. The mathematics behind this are directly related to the arithmetic-geometric mean inequality and the Cauchy–Schwarz inequality.[9]

Formally, Feld assumes that a social network is represented by an undirected graph G = (V, E), where the set V of vertices corresponds to the people in the social network, and the set E of edges corresponds to the friendship relation between pairs of people. That is, he assumes that friendship is a symmetric relation: if X is a friend of Y, then Y is a friend of X. He models the average number of friends of a person in the social network as the average of the degrees of the vertices in the graph. That is, if vertex v has d(v) edges touching it (representing a person who has d(v) friends), then the average number μ of friends of a random person in the graph is

${\displaystyle \mu ={\frac {\sum _{v\in V}d(v)}{|V|}}={\frac {2|E|}{|V|}}.}$

The average number of friends that a typical friend has can be modeled by choosing a random person (who has at least one friend), and then calculating how many friends their friends have on average. This amounts to choosing, uniformly at random, an edge of the graph (representing a pair of friends) and an endpoint of that edge (one of the friends), and again calculating the degree of the selected endpoint. The probability of a certain vertex ${\displaystyle v}$ to be chosen is :

${\displaystyle {\frac {d(v)}{|E|}}{\frac {1}{2}}}$

The first factor corresponds to how likely it is that the chosen edge contains the vertex, which increases when the vertex has more friends. The halving factor simply comes from the fact that each edge has two vertices. So the expected value of the number of friends of a (randomly chosen) friend is :

${\displaystyle \sum _{v}\left({\frac {d(v)}{|E|}}{\frac {1}{2}}\right)d(v)={\frac {\sum _{v}d(v)^{2}}{2|E|}}}$

We know from the definition of variance that :

${\displaystyle {\frac {\sum _{v}d(v)^{2}}{|V|}}=\mu ^{2}+\sigma ^{2}}$

where ${\displaystyle \sigma ^{2}}$ is the variance of the degrees in the graph. This allows us to compute the desired expected value :

${\displaystyle {\frac {\sum _{v}d(v)^{2}}{2|E|}}={\frac {|V|}{2|E|}}(\mu ^{2}+\sigma ^{2})={\frac {\mu ^{2}+\sigma ^{2}}{\mu }}=\mu +{\frac {\sigma ^{2}}{\mu }}}$

For a graph that has vertices of varying degrees (as is typical for social networks), ${\displaystyle {\sigma }^{2}}$ is strictly positive, which implies that the average degree of a friend is strictly greater than the average degree of a random node.

Another way of understanding how the first term came is as follows. For each friendship (u, v), a node u mentions that v is a friend and v has d(v) friends. There are d(v) such friends who mention this. Hence the square of d(v) term. We add this for all such friendships in the network from both the u's and v's perspective, which gives the numerator. The denominator is the number of total such friendships, which is twice the total edges in the network (one from the u's perspective and the other from the v's).

After this analysis, Feld goes on to make some more qualitative assumptions about the statistical correlation between the number of friends that two friends have, based on theories of social networks such as assortative mixing, and he analyzes what these assumptions imply about the number of people whose friends have more friends than they do. Based on this analysis, he concludes that in real social networks, most people are likely to have fewer friends than the average of their friends' numbers of friends. However, this conclusion is not a mathematical certainty; there exist undirected graphs (such as the graph formed by removing a single edge from a large complete graph) that are unlikely to arise as social networks but in which most vertices have higher degree than the average of their neighbors' degrees.

Applications

The analysis of the friendship paradox implies that the friends of randomly selected individuals are likely to have higher than average centrality. This observation has been used as a way to forecast and slow the course of epidemics, by using this random selection process to choose individuals to immunize or monitor for infection while avoiding the need for a complex computation of the centrality of all nodes in the network.[10][11][12]

A study in 2010 by Christakis and Fowler showed that flu outbreaks can be detected almost 2 weeks before traditional surveillance measures can by using the friendship paradox in monitoring the infection in a social network.[13] They found that using the friendship paradox to analyze the health of central friends is "an ideal way to predict outbreaks, but detailed information doesn't exist for most groups, and to produce it would be time-consuming and costly."[14]

The "generalized friendship paradox" states that the friendship paradox applies to other characteristics as well. For example, one's co-authors are on average likely to be more prominent, with more publications, more citations and more collaborators,[15][16][17] or one's followers on Twitter have more followers.[18] The same effect has also been demonstrated for Subjective Well-Being by Bollen et al (2017),[19] who used a large-scale Twitter network and longitudinal data on subjective well-being for each individual in the network to demonstrate that both a Friendship and a "happiness" paradox can occur in online social networks.

See also

• Second neighborhood problem
• Self-evaluation maintenance theory

References

1. Feld, Scott L. (1991), "Why your friends have more friends than you do", American Journal of Sociology, 96 (6): 1464–1477, doi:10.1086/229693, JSTOR 2781907.
2. Zuckerman, Ezra W.; Jost, John T. (2001), "What makes you think you're so popular? Self evaluation maintenance and the subjective side of the "friendship paradox"" (PDF), Social Psychology Quarterly, 64 (3): 207–223, doi:10.2307/3090112, JSTOR 3090112.
3. McRaney, David (2012), You are Not So Smart, Oneworld Publications, p. 160, ISBN 978-1-78074-104-8
4. Felmlee, Diane; Faris, Robert (2013), "Interaction in social networks", in DeLamater, John; Ward, Amanda (eds.), Handbook of Social Psychology (2nd ed.), Springer, pp. 439–464, ISBN 978-9400767720. See in particular "Friendship ties", p. 452.
5. Lau, J. Y. F. (2011), An Introduction to Critical Thinking and Creativity: Think More, Think Better, John Wiley & Sons, p. 191, ISBN 978-1-118-03343-2
6. Kanazawa, Satoshi (2009), "The Scientific Fundamentalist: A Look at the Hard Truths About Human Nature – Why your friends have more friends than you do", Psychology Today, archived from the original on 2009-11-07.
7. Burkeman, Oliver (30 January 2010), "This column will change your life: Ever wondered why your friends seem so much more popular than you are? There's a reason for that", The Guardian.
8. Lerman, Kristina; Yan, Xiaoran; Wu, Xin-Zeng (2016-02-17). "The "Majority Illusion" in Social Networks". PLOS ONE. 11 (2): e0147617. arXiv:1506.03022. Bibcode:2016PLoSO..1147617L. doi:10.1371/journal.pone.0147617. ISSN 1932-6203. PMC 4757419. PMID 26886112.
9. Ben Sliman, Malek; Kohli, Rajeev (2019), "The extended directed friendship paradox", SSRN, doi:10.2139/ssrn.3395317, S2CID 219376223
10. Cohen, Reuven; Havlin, Shlomo; ben-Avraham, Daniel (2003), "Efficient immunization strategies for computer networks and populations", Phys. Rev. Lett., 91 (24), 247901, arXiv:cond-mat/0207387, Bibcode:2003PhRvL..91x7901C, doi:10.1103/PhysRevLett.91.247901, PMID 14683159.
11. Christakis, N. A.; Fowler, J. H. (2010), "Social network sensors for early detection of contagious outbreaks", PLOS ONE, 5 (9), e12948, arXiv:1004.4792, Bibcode:2010PLoSO...512948C, doi:10.1371/journal.pone.0012948, PMC 2939797, PMID 20856792.
12. Wilson, Mark (November 2010), "Using the friendship paradox to sample a social network", Physics Today, 63 (11): 15–16, Bibcode:2010PhT....63k..15W, doi:10.1063/1.3518199.
13. Christakis, Nicholas A.; Fowler, James H. (September 15, 2010). "Social Network Sensors for Early Detection of Contagious Outbreaks". PLOS ONE. 5 (9): e12948. arXiv:1004.4792. Bibcode:2010PLoSO...512948C. doi:10.1371/journal.pone.0012948. PMC 2939797. PMID 20856792.
14. Schnirring, Lisa (Sep 16, 2010). "Study: Friend 'sentinels' provide early flu warning". CIDRAP News.
15. Eom, Young-Ho; Jo, Hang-Hyun (2014), "Generalized friendship paradox in complex networks: The case of scientific collaboration", Scientific Reports, 4, 4603, arXiv:1401.1458, Bibcode:2014NatSR...4E4603E, doi:10.1038/srep04603, PMC 3980335, PMID 24714092
16. Grund, Thomas U. (2014), "Why Your Friends Are More Important And Special Than You Think" (PDF), Sociological Science, 1: 128–140, doi:10.15195/v1.a10
17. Dickerson, Kelly. "Why Your Friends Are Probably More Popular, Richer, and Happier Than You". Slate Magazine. The Slate Group. Retrieved 17 January 2014.
18. Hodas, Nathan; Kooti, Farshad; Lerman, Kristina (May 2013). "Friendship Paradox Redux: Your Friends are More Interesting than You". arXiv:1304.3480 [cs.SI].
19. Bollen, Johan; Goncalves, Bruno; Van de Leemput, Ingrid; Guanchen, Ruan (2017), "The happiness paradox: your friends are happier than you", EPJ Data Science, 6, arXiv:1602.02665, doi:10.1140/epjds/s13688-017-0100-1, S2CID 2044182

External links

• Strogatz, Steven (September 17, 2012). "Friends You Can Count On". New York Times. Retrieved 17 January 2013.